1. Field of the Invention
The present invention relates to a control device for a die cushion mechanism which generates a force on a slide of a press machine using a servomotor as a driving source and, in particular, to a control device for controlling the force generated by the die cushion mechanism of the press machine.
2. Description of the Related Art
It is known that a press machine, for press working such as bending, drawing or punching, is provided with a die cushion mechanism as an attached device for applying a predetermined force or pressure, during the press working, to a movable support member (generally called a slide) supporting a first mold for press working, the force being generated by another movable member supporting a second mold. The die cushion mechanism is generally configured such that the slide (or the first mold), moving in a mold-clamping direction, directly or indirectly collides with a movable element (generally called as a cushion pad) held at a predetermined pressure and, until the molding is finished, the cushion pad is moved with the slide while applying force or pressure to the slide. During this operation, it is possible to prevent the occurrence of a wrinkle in a workpiece to be pressed by, for example, clamping an area around a site, of the workpiece, to be pressed between the cushion pad and the slide.
Many conventional die cushion mechanisms use a hydraulic or a pneumatic unit as a driving source. However, the control by the hydraulic or the pneumatic unit may be carried out only under a constant pressure. It is preferable that the pressure during drawing is varied corresponding to the amount of the drawing, however, the pressure cannot be varied in the hydraulic or the pneumatic unit.
In recent years, therefore, a die cushion mechanism using a servomotor as a driving source has been used to carry out force control with a high response, as described in Japanese Unexamined Patent Publication (Kokai) No. 10-202327. In the die cushion mechanism described in this publication, a cushion pad positioned below a slide of a press machine may be upwardly and downwardly moved by a servomotor, corresponding to the rise and fall motions of the slide. The servomotor is activated by a predetermined force command corresponding to the position of the cushion pad and adjusts the force or pressure applied, to the slide, by the cushion pad while moving the cushion pad with the slide. The collision of, and pressure between, the slide and the cushion pad may be determined by detecting a load applied to an output axis of the servomotor via the cushion pad.
In recent press working, it is desired to carry out the press working with a higher accuracy, and at a high speed, in order to improve the productivity. In order to increase a speed of the press working, it is necessary to rapidly accelerate or decelerate the slide during the press working. Also, in controlling the force of the die cushion mechanism, a necessary torque for controlling the force is obtained, as well as another torque for accelerating or decelerating the die cushion such that the motion of the die cushion may follow the motion of the slide. In driving a servomotor in the prior art, a response of the force control is constant without relation to the acceleration or the deceleration of the slide. Therefore, when the slide is gradually accelerated or decelerated, the force may be controlled so as to follow a commanded force. However, when the slide is rapidly accelerated or decelerated, the force is deviated from the commanded force. On the other hand, when a force gain is increased for the rapid acceleration or deceleration of the slide, a problem such as vibration at a low frequency may occur in the machine during the gradual acceleration or deceleration of the slide. Due to this, it is difficult to increase the force gain for rapid acceleration or deceleration.